The present invention relates to an ion detector system for a mass spectrometer, a mass spectrometer, a method of detecting ions, a method of mass spectrometry, a time of flight mass analyser, a method of operating a time of flight mass analyser, a photon detection system, a streak camera detector and a method of detecting photons. The preferred embodiment relates to an ion detector system for a Time of Flight mass spectrometer.
Streak cameras are known and are versatile detection instruments for detecting light in a manner which gives temporal information. It is known, for example, to use a streak camera to measure electron bunches in synchrotrons and also to measure ultra fast (i.e. femtosecond) laser pulses. It is also known to use streak cameras in plasma physics experiments.
Conventional streak cameras convert pulsed incident light to electrons using a photocathode. The electrons which are emitted from the photocathode are then accelerated by a mesh electrode. The electrons are then arranged to pass between two parallel plate electrodes. A temporally varying sweep voltage is applied to the plate electrodes such that the electrons receive a deflection. The electrons are then arranged to impinge upon a phosphorescent screen.
The signal from the phosphorescent screen is read by a position sensitive detector (“PSD”). The position of the signal on the screen is directly related to the instantaneous sweep voltage that the electrons encountered when the electrons passed rapidly between the two deflection plates whilst a sweep voltage was being applied to the deflection plates.
It will be apparent that streak cameras operate by converting temporal information into spatial information. As a result, depending upon the application, this process reduces the requirements placed upon high speed digitising electronics which are used to capture the signal and/or increases the overall temporal resolution of the detector.
The streak camera principle has been applied to the detection of heavy ions in the radioactive ion beam research facility at RIKEN (Japan). Two streak cameras were used to register secondary electrons produced by a heavy highly energetic ion passing through thin metallic films. The device employed 100 MHz waveforms to deflect an ion beam in x- and y-dimensions onto a phosphor screen. The phosphorescence was then amplified and captured by a Charged Coupled Device (“CCD”) camera.
WO 2012/010894 (ISIS) discloses a charged particle spectrum analysis apparatus. FIG. 1 of WO 2012/010894 shows an arrangement wherein sample ions 5 impinge upon a first set of microchannel plates 3 converting the ions into an amplified beam of electrons 7. The beam of electrons 7 is collimated by a slit 9 placed behind the microchannel plates 3. Electrons 7 emitted from the microchannel plates 3 are accelerated through the slit 9 and are subjected to a ramped deflection pulse by two parallel deflection plates 11. The electrons 7 are deflected onto a position sensitive detector 15. The ramp voltage is synchronised to the frame rate of a CMOS sensor which is set at 20 MHz (corresponding with a bin width 50 ns).
As digital electronics gets faster so the digitisation frequency of Time to Digital Converters and Analogue to Digital Converters is also predicted to increase. As a result, the deflection plates of conventional streak cameras would need to be swept at increasingly faster rates.
However, as will be understood by those skilled in the art, it is non-trivial to increase the frequency of the deflection voltage waveforms. Furthermore, it will be appreciated that producing an intense electric field for sweeping electrons in a streak camera in synchronisation with current state of the art digitisers capable of running at bin widths of e.g. 100 ps or faster is technically extremely challenging and the cost would render any such detector system commercially unviable.
Accordingly, it is highly problematic to design a low power, cost effective and stable means of deflecting electrons using deflection electrodes in combination with a position sensitive array detector which is capable of operating at the very high speeds which are required by state of the art detector systems for mass spectrometers.
It is therefore desired to provide an improved ion detector system and an improved method of detecting ions.